1. Field of the Invention
This invention relates to a fail-fixed servovalve and, more particularly, to an improved, simplified fail-fixed servovalve which is particularly suitable for use in conjunction with a pulse width modulated digital driving signal.
2. Description of the Prior Art
Servovalves of the electrohydraulic type have been widely used as an interface between electrical control systems and different types of mechanical or hydraulic metering or actuating devices. For example, in a gas turbine engine fuel control system, an electrical control signal generated by an electronic fuel control computer may be applied to the input of a servovalve. In response to the electrical input signal, the servovalve controls the movement of a servopiston which translates within a bore to generate a mechanical output signal for varying the position of a mechanical fuel metering valve. Thus, the flow of fuel to the gas turbine engine can be accurately controlled as a function of the computer generated electrical signal.
Due to the widespread use of such servovalves in particularly critical control systems, such as the above-described gas turbine engine fuel control system, it is desirable for the servovalve to be fail-fixed. By fail-fixed, it is meant that the mechanical output of the servovalve is zero (i.e., the servopiston is locked in position) in the event the electrical input signal is either lost or exceeds a rated maximum value. An example of such a fail-fixed servovalve is described in U.S. Pat. No. 3,922,955, assigned to the assignee of the present invention.
The prior art fail-fixed servovalve described in the aforementioned U.S. Patent operates utilizing bi-polar input currents; that is, the servopiston moves in one direction when positive current is applied to the servovalve input and moves in the opposite direction when negative current is applied to the servovalve input. For zero or null current and for a surrounding deadband region on both the positive and negative sides of zero current, the servopiston is essentially locked in position (fixed) with slight movements due to fluid leakage. Although this type of fail-fixed servovalve has proven to be adequate for many applications, the inherent deadband at null requires a complex driver circuit with deadband compensation.
The prior art fail-fixed servovalve is also relatively expensive to produce and the use of a contamination sensitive second stage spool makes it more expensive to operate due to the need for additional fluid filtration.
It is, therefore, an object of the present invention to provide an improved, simplified fail-fixed servovalve which is compatible with pulse width modulated digital driving signals.
It is another object of the present invention to provide such fail-fixed servovalve which is simple to operate and relatively inexpensive to produce.
It is a further object of the present invention to provide such a fail-fixed servovalve which does not require additional deadband compensation circuitry.
It is yet another object of the present invention to provide such a fail-fixed servovalve which is less sensitive to fluid contamination.